This invention relates to a catalyst and a process for using such a catalyst to dewax a waxy hydrocarbon feedstock containing straight and branched chain paraffins. The invention is particularly concerned with a process for producing a hydrocarbon product having a relatively low pour point and/or cloud point from a full boiling range shale oil.
Many liquid hydrocarbon feedstocks contain relatively high concentrations of straight and branched chain aliphatic compounds having between 6 and 40 carbon atoms. Some of these long chain compounds, typically the ones containing 12 or more carbon atoms, tend to crystallize upon cooling of the hydrocarbon oil. This crystallization may only take place to the extent that a clear and bright oil becomes dull because of the formation of small crystals, or it may proceed until sufficient crystals are present that they interfere with the flow of the hydrocarbon liquid from one location to another. The temperature at which the waxy substances begin to crystallize from the solution and impart a cloudy appearance is referred to as the "cloud point" and is determined by standardized test procedures. Similarily, the temperature at which sufficient crystals form to prevent the hydrocarbon oil from flowing is commonly referred to as the "pour point" and is also determined by standardized test procedures. An example of a feedstock having a relatively high pour point is the raw oil obtained by retorting oil shale, such as the oil shale found in the Colorado River formation in the Western United States.
Oil shale is a mixture of a minor amount of solid organic matter known as kerogen and a major amount of mineral matter. One method of recovering hydrocarbons from oil shale is by subjecting the shale to heat via pyrolysis or retorting at temperatures between about 850.degree. F. and about 1,000.degree. F. These high temperatures cause the kerogen to decompose into liquid and light gaseous hydrocarbonaceous products. The liquids recovered by condensing the oil shale vapors will normally contain a relatively high concentration of straight and branched chain paraffins of high molecular weight. This high concentration of waxy components typically results in the oil having a relatively high pour point, normally between about 50.degree. F. and about 90.degree. F. In addition, the raw shale oil will contain arsenic, organonitrogen constituents and/or organosulfur constituents.
Raw shale oil is normally upgraded by contacting the oil with a hydrotreating catalyst under conditions such that the concentration of organosulfur and organonitrogen constituents is reduced. The hydrotreated shale oil may then be dewaxed by contacting the oil with a dewaxing catalyst under dewaxing conditions to remove straight and slightly branched chain paraffins and thereby reduce the pour point. In some cases it may be desirable to use the portion of the dewaxed product which boils above about 600.degree. F. as lube oil base stock. This portion of the dewaxed shale oil can be thought of as comprising six lube oil base stock fractions which boil in the following ranges: 610.degree. F. to 650.degree. F., 650.degree. F. to 690.degree. F., 690.degree. F. to 790.degree. F., 790.degree. F. to 830.degree. F., 830.degree. F. to 875.degree. F. and above 875.degree. F. In order for the portion of dewaxed shale oil boiling above 600.degree. F. to be suitable as lube oil base stock, each of the boiling fractions described above must meet minimum pour point requirements. Normally, the pour point must be 10.degree. F. or lower for each of the fractions. Unfortunately, the pour point of a fraction increases as the boiling range of the fraction increases. In conventional dewaxing processes that utilize catalysts which contain an intermediate pore molecular sieve, it is very difficult to meet the pour point requirements for the fractions boiling between 790.degree. F. and 830.degree. F., between 830.degree. F. and 875.degree. F. and above 875.degree. F. In order to meet the minimum pour point requirements of these higher boiling fractions, the dewaxing temperature must either be increased, the space velocity through the dewaxing zone decreased, or a more active catalyst used. Although the use of any one of these three techniques will decrease the pour point of the higher boiling fractions, such use will also result in more conversion of the branched and straight chain paraffins in the lower boiling fractions than is necessary to meet the pour point requirements of these fractions. This additional conversion of branched and straight chain paraffins in the lower boiling fractions represents significant losses in yield of lube oil base stock.
Accordingly, it is one of the objects of the present invention to provide a process for dewaxing shale oil and other waxy hydrocarbon feedstocks such that the various boiling fractions comprising the lube oil base stock portion of the dewaxed oil meet minimum pour point requirements. It is another object of the invention to provide a dewaxing catalyst which can be used to dewax shale oil and other waxy hydrocarbon feedstocks under conditions such that the pour point requirements of all the boiling fractions comprising the lube oil base stock portion of the dewaxed oil are met without incurring excessive losses of lube oil base stock. It is yet another object of the invention to provide a catalyst and process which will remove both straight and branched chain paraffins from waxy hydrocarbon feedstocks. These and other objects of the invention will become more apparent in light of the following description of the invention.